The present invention relates to a flow control valve used for various industrial instruments, for example, a hydraulically control apparatus, such as a vehicle and a forklift.
Japanese Patent Publication (KOKAI) No. 5-204465 has disclosed conventional pressure compensation type flow control valves as shown in FIGS. 6 and 7. The flow control valve shown in FIG. 6 has a structure in which a spool 22 is slidably fitted in a cylindrical body 21, and a spring 24 is disposed between the spool 22 and a control screw portion 23 screwed to a base end of the body 21. When a fluid flows from an inflow port 26 to an outflow port 25 along an arrow in FIG. 6, the spool 22 performs a pressure-compensation operation for automatically controlling an opening of a throttle portion 22a formed between the spool 22 and the body 21. Accordingly, the fluid flows out of the outflow port 25 at a constant rate corresponding to an opening of a fixed orifice 27 disposed at the inflow port 26. In this case, the control screw portion 23 is rotated for adjusting a spring force of the spring 24 to control the flow rate.
The flow control valve as shown in FIG. 7 has a structure in which an inner cylinder 32 as a cylindrical portion is fitted in an outer cylinder 31. An annular space 34 is formed between the outer cylinder 31 and the inner cylinder 32 at a forward end side thereof from a screwing portion 33. Circular opening portions 31a, 32a are formed in peripheral walls of the outer cylinder 31 and the inner cylinder 32 for communicating inside portions of the outer cylinder 31 and the inner cylinder 32 with outside portions thereof, respectively. An inner periphery of the inner cylinder 32 is communicated with an outer periphery of the outer cylinder 31 through the opening portion 32a, the annular space 34 and the opening portion 31a. A cylindrical spool 35 is slidably fitted in the annular space 34.
The spool 35 has a circular opening portion 35b at a position adjacent to a land portion 35a. When the spool 35 is displaced in an axial direction, a throttle portion 36 formed between an opening portion 35b and the opening portion 31a of the outer cylinder 31 is changed from a fully closed state to a fully opened state. A spring 37 is provided between a right end side of the spool 35 and an inner end of the screwing portion 33 to thereby elastically contact them, so that the spool 35 is elastically urged toward the left end side by the spring 37.
A piston 38 operated externally is slidably fitted in the inner periphery of the inner cylinder 32. The piston 38 includes a screw portion 38a integrally, and is movably screwed into the inner cylinder 32 through the screw portion 38a to thereby form a variable orifice portion 40 between the opening portion 32a of the inner cylinder 32 and the piston 38.
When the fluid flows in through an inlet port 41a, the fluid flows into the annular space 34 through the orifice portion 40 from the inner periphery of the inner cylinder 32 along an arrow in FIG. 7 (hidden line). The fluid flows out to the outlet port 41b through the throttle portion 36 at a constant flow rate. In this case, the piston 38 is moved for changing an area of the opening portion 32a to control the flow rate.
In the conventional flow control valves with the structures described above, when the fluid (operating oil) flows into the flow control valve, a fluid force is applied to the spool. Therefore, there is a difference in pressures at a front side and a rear side of the flow control valve. When the fluid force becomes too large, the fluid flows into the flow control valve at a very low rate. As a result, it is difficult for the flow control valve to perform the pressure-compensation function.
In view of the above problems, the present invention has been made, and an object of the invention is to provide a flow control valve capable of maintaining a predetermined flow rate regardless of a difference in pressures at a front side and a rear side of the flow control valve.
Further objects and advantages of the invention will be apparent from the following description of the invention.